A known electronic engine control system comprises a processor-based controller that processes data from various sources to develop control data for controlling certain aspects of engine operation such as speed and output torque. Control of speed and output torque of a diesel engine is in large part accomplished by controlling how the engine is fueled, but also in conjunction with control of other factors that include engine boost and back pressure, exhaust gas recirculation (EGR), and in an engine equipped with variable valve timing, even compression ratio.
Control of engine fueling comprises controlling the quantity of fuel injected into an engine cylinder during a fuel injection and controlling the timing of the injection. State-of-the-art fuel injector systems and associated electronics enable engine fueling to be controlled with precision. The other factors mentioned above can also be controlled with precision through the use of state-of-the-art devices and associated electronics.
While control of individual devices, like fuel injectors, EGR valves, or turbocharger vanes for example, can be accomplished with precision, the cumulative effect of controlling each individual device when the engine is in a dynamic state of operation may at times tend to cause temporary disturbances such as transient oscillations, perturbations, overshoots, and the like, in certain operating parameters like engine speed and torque. It is of course desirable that such disturbances be avoided, or at least minimized, not only for the sake of engine performance, but also because they may have adverse consequences on tailpipe emissions.
A diesel engine controller is typically calibrated with set-point values for certain parameters that characterize the engine and how it should be operated. One such parameter is engine high idle speed. High idle speed is the maximum engine speed that the controller will allow and at that speed, flywheel torque is zero, meaning that the controller is causing the engine to run at that speed while developing only enough torque to overcome friction and pumping losses so that no output torque is available at the flywheel. Typical high idle speeds for diesel engines used in trucks are in the range from about 2,000 rpm to about 3,000 rpm. It is important for an engine controller to assure that engine operation doesn't exceed high idle speed.
A known controller for engine speed uses a PI (proportional-integral) control strategy whose intent is to secure faithful correspondence of engine speed to an engine speed set-point that can have any value within the engine's speed range. When engine high idle becomes the set-point, it is important that the controller keep engine speed from exceeding the set-point.
It has been discovered engine speed may not always be controlled with repeatable accuracy in certain situations and that some instability and/or loss of accuracy may become noticeable. One example of this was observed in an engine whose high idle set-point changed as a function of transmission gear selection in an automatic transmission equipped vehicle.
During operation of a vehicle on the road, assured limiting of engine speed by the engine controller becomes especially important when a controller other than the engine controller is requesting that engine speed be limited. Transmission, ABS, and traction controllers that interact with an engine controller are examples of such other controllers.